1. Field of the Invention
The invention relates to heat exchangers and, more particularly, relates to an improved finned tube-type heat exchanger and to a method of making the same.
2. Discussion of the Related Art
Finned tube heat exchangers are well known for exchanging heat between fluid flowing through tubes and an ambient fluid surrounding the tubes. The typical finned tube heat exchanger includes (1) a plurality of parallel fins formed from thin sheets of aluminum or another thermally conductive material and (2) a plurality of parallel tubes extending through apertures in the fins and formed from copper or another thermally conductive metal. The tubes are expanded against collars surrounding the apertures to provide a firm mechanical connection between the fins and tubes and to enhance heat exchange by conduction between the tubes and fins.
Referring now to FIGS. 1-3, a finned tube heat exchanger 10 is typically constructed by first punching blanks 12 out of an aluminum sheet 14 to form apertures 16 (FIG. 1), expanding the apertures 16 to form collars 18 (FIG. 2), and then inserting tubes 20 through the apertures 16 and expanding the tubes 20 into the collars 18 (FIG. 3).
Forming apertures in the sheets 14 by removing blanks 12 exhibits several drawbacks and disadvantages both during manufacturing and in use. During manufacturing, the blanks 12 tend to litter the work area and frequently jam the fin press and related machinery. In use, performance of the heat exchanger 10 is significantly degraded because the surface area of the blanks 12, which would otherwise be available for heat exchange, is lost when the blanks 12 are punched out of the sheets 14. The heat exchange capacity of a particular fin construction varies with available surface area. Hence, completely removing the blanks significantly decreases the overall heat exchange efficiency of a heat exchanger. In a typical finned tube heat exchanger using 3/8" tubes about 14% of the available fin surface is lost when the blanks are removed, with a proportionate decrease in heat exchange capacity. This lost available surface area increases to 17% for heat exchangers using 1/2" tubes, with a further decrease in heat exchange capacity.
Proposals have been made to increase the heat exchange efficiency of finned tube heat exchangers. For instance, U.S. Pat. No. 5,042,576 to Broadbent (the Broadbent patent) recognizes that heat exchange capacity is higher at relatively high temperature differentials and decreases with decreasing temperature differentials. The Broadbent patent attempts to increase the heat exchange capacity of a finned tube heat exchanger of designated overall dimensions by increasing the surface area of the fin assembly which contacts streams of ambient fluid which are at or near ambient temperature. This surface area is increased by deforming the major surface area of the fins into raised louvers or lances which extend at different levels with respect to each other and with respect to the major surfaces of the fins and which accordingly contact different airstreams flowing through the heat exchanger.
The Broadbent patent also recognizes that the overall efficiency of a heat exchanger depends not only on the rate of heat exchange, but also on the cost of forcing air through the heat exchanger. The Broadbent patent attempts to minimize this cost by maintaining a low pressure drop across the heat exchanger through the use of louvers which are relatively flat and which extend in parallel with the direction of airflow.
The raised lance or louvered finned tube heat exchanger proposed by Broadbent, though more efficient than heat exchangers employing only planar fins, is relatively expensive to fabricate and to install because the louvers must be formed in the fins. Moreover, because the apertures for receiving the tubes are formed by punching blanks out of the fins, the surface area of these apertures is lost for heat exchange purposes, with a resultant and proportional decrease in heat exchange capacity. The increased heat exchange capacity resulting from the raised lances or louvers is thus at least partially offset by the lost fin surface area at the apertures.
U.S. Pat. Nos. 1,634,110 to McIntyre, 2,089,340 to Cobb, 3,190,353 to Storfer, 3,384,168 to Richter, and 5,117,905 to Hesse all disclose heat exchangers in which some of the materials from the apertures of heat exchange fins is preserved. However, the materials preserved in the heat exchanger of each of these patents is used to facilitate the mounting of the fins on the tubes (see McIntyre, Cobb and Storfer), and/or to set the spacing between adjacent fins (see Richter and Hesse). Even those patents which recognize an increase in heat exchange capacity from such structures merely attempt to increase heat exchange capacity by increasing the surface contact area between the tubes and the fins, and not by forming secondary heat exchange surfaces operating at least generally in parallel to the main fin surfaces.